Medical devices of the kind to which the present invention relates are used in positioning bone parts with respect to each other in case of a fracture or if two bones that are connected via one or more joints are to be immobilised. In all these cases the medical devices of the present kind are intended to take up and transmit a load in a direction substantially transversely to the longitudinal direction of the medical device.
Medical devices intended for fixing broken bone parts relative to each other are used in particular in the treatment of a broken femur head. An example of known medical devices of this kind is to be found in European patent EP 0 337 288 A1. According to said document, one or more screws are screwed into the femur head, which screw is slidingly (in only one direction) accommodated in the plate that is fixed to the femur by means of screws. The screw and the plate are rigid, to such an extent that the device is capable of transmitting loads resulting from walking movements in a direction transversely to the longitudinal axis of the screw, wherein the screw can move at a fixed angle in the direction of the femur, as a result of which the broken bones are firmly pressed together in the plane of fracture.
Another example of the known medical devices is to be found in U.S. Pat. No. 0,004,514 A1, for example, which employs a pin that is placed in the intramedullary cavity of the upper leg. A problem that occurs with these known devices is caused by the fact that the femur head can rotate freely about the screw axis with respect to the screw, in any case during insertion, which may lead to necrosis of the femur head. In addition, placement of the screw may cause damage to the bone in the femur head, to such an extent that it seriously impedes the healing process.
Medical devices that attempt to obviate this problem are known from U.S. Pat. No. 3,561,437, in which the pin is slidingly accommodated in one direction in the plate and the pin is driven rather than screwed into the bone. A problem that occurs with these known devices is caused by the fact that the pin can easily move out of the femur head.
To obviate this problem, constructions consisting of a pin that is slidingly accommodated in a plate have been conceived, in which constructions the pin is provided with elements that can expand in a direction transversely to the longitudinal direction of the pin. Examples of such constructions are shown in U.S. Pat. Nos. 2,699,774, 2,397,545 and 4,204,531, for example. With all these constructions, the end of the pin that faces towards the femur head is provided with elements that are movable in the transverse direction of the pin, which parts elements are moved apart by suitable means once the pin has been placed. All these suitable means are fairly complicated and thus costly. A drawback of these known devices is that they are not hollow, so that they cannot be inserted over a guide wire.
Medical devices intended for use in situations in which two bones connected by a joint are to be immobilised are used in particular in fixing a vertebral segment. An example of known medical devices of this kind is to be found in U.S. Pat. No. 5,209,753, in which a screw is inserted into the pedicles of two or more vertebrae, which vertebrae are connected by means of plates or rods.
A problem that occurs with these known devices is caused by the fact that part of the bone in the pedicle may be damaged, to such an extent that a proper fixation is not obtained. In addition, very dangerous situations may arise upon turning a screw into the vertebra, because fibres of the dura may get entangled when the screw breaks through the pedicle wall on the side of the spinal cord.